Realizing Concurrency using the thread modelIntroductionTopics to be CoveredObjectivesThe Thread ModelPer process vs per thread itemsImplementing Threads in User SpaceImplementing Threads in the KernelHybrid ImplementationsScheduler ActivationsPop-Up ThreadsThread Scheduling (1)Thread Scheduling (2)Thread as a unit of workPthread LibraryPosix Library Implementation in F. Mueller’s PaperCreating threadsUsing threadsThread’s local dataThread termination (destruction)Waiting for thread exitSummary01/14/191Realizing Concurrency using the thread model B. Ramamurthy01/14/192IntroductionA thread refers to a thread of control flow: an independent sequence of execution of program code.Threads are powerful. As with most powerful tools, if they are not used appropriately thread programming may be inefficient.Thread programming has become viable solution for many problems with the advent of multiprocessors and client-server model of computing.Typically these problems are expected to handle many requests simultaneously. Example: multi-media, database applications, web applications.01/14/193Topics to be CoveredObjectivesWhat are Threads?Thread implementation modelsPOSIX threadsCreating threadsUsing threadsSummary01/14/194ObjectivesTo understand the thread model for realizing concurrencyTo study POSIX standard for threads called Pthreads.To study thread control primitives for creation, termination, join, synchronization, concurrency, and scheduling.To learn to design multi-threaded applications.01/14/195The Thread Model(a) Three processes each with one thread(b) One process with three threads01/14/196Per process vs per thread itemsItems shared by all threads in a processItems private to each thread01/14/197Implementing Threads in User SpaceA user-level threads package01/14/198Implementing Threads in the KernelA threads package managed by the kernel01/14/199Hybrid Implementations Multiplexing user-level threads onto kernel- level threads01/14/1910Scheduler ActivationsGoal – mimic functionality of kernel threadsgain performance of user space threadsAvoids unnecessary user/kernel transitionsKernel assigns virtual processors to each processlets runtime system allocate threads to processorsProblem: Fundamental reliance on kernel (lower layer) calling procedures in user space (higher layer)01/14/1911Pop-Up ThreadsCreation of a new thread when message arrives(a) before message arrives(b) after message arrivesThread pools01/14/1912Thread Scheduling (1)Possible scheduling of user-level threads50-msec process quantumthreads run 5 msec/CPU burstB1, B2, B301/14/1913Thread Scheduling (2)Possible scheduling of kernel-level threads50-msec process quantumthreads run 5 msec/CPU burstB1, B2, B301/14/1914Thread as a unit of workA thread is a unit of work to a CPU. It is strand of control flow.A traditional UNIX process has a single thread that has sole possession of the process’s memory and resources.Threads within a process are scheduled and execute independently.Many threads may share the same address space.Each thread has its own private attributes: stack, program counter and register context.01/14/1915Pthread LibraryMany thread models emerged: Solaris threads, win-32 threadsA POSIX standard (IEEE 1003.1c) API for thread creation and synchronization.API specifies behavior of the thread library, implementation is up to development of the library.Simply a collection of C functions.One of the undergraduates from UB implemented a object-oriented version of the Pthreads called Zthreads: /projects/bina/zthread-1.5.101/14/1916Posix Library Implementationin F. Mueller’s PaperLanguage ApplicationLanguage Interface C Language ApplicationPosix thread libraryUnix KernelUnix librariesUser LevelKernel Level01/14/1917Creating threadsAlways include pthread library: #include <pthread.h>int pthread_create (pthread_t *tp, const pthread_attr_t * attr, void *(* start_routine)(void *), void *arg);This creates a new thread of control that calls the function start_routine.It returns a zero if the creation is successful, and thread id in tp (first parameter).attr is to modify the attributes of the new thread. If it is NULL default attributes are used.The arg is passing arguments to the thread function.01/14/1918Using threads1. Declare a variable of type pthread_t2. Define a function to be executed by the thread.3. Create the thread using pthread_createMake sure creation is successful by checking the return value.4. Pass any arguments need through’ arg (packing and unpacking arg list necessary.)5. #include <pthread.h> at the top of your header.6. Compile: g++ -o executable file.cc -lpthread01/14/1919 Thread’s local dataVariables declared within a thread (function) are called local data.Local (automatic) data associated with a thread are allocated on the stack. So these may be deallocated when a thread returns. So don’t plan on using locally declared variables for returning arguments. Plan to pass the arguments thru argument list passed from the caller or initiator of the thread.01/14/1920Thread termination (destruction)Implicit : Simply returning from the function executed by the thread terminates the thread. In this case thread’s completion status is set to the return value.Explicit : Use thread_exit. Prototype: void thread_exit(void *status);The single pointer value in status is available to the threads waiting for this thread.01/14/1921Waiting for thread exitint pthread_join (pthread_t tid, void * *statusp);A call to this function makes a thread wait for another thread whose thread id is specified by tid in the above prototype.When the thread specified by tid exits its completion status is stored and returned in statusp.01/14/1922SummaryWe looked at Implementation of threads.thread-based concurrency.Pthread programmingWe will look at a pthread programming demoStudy the details given in thread library link.See
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